1. Field of the Invention
The present invention relates to substrates with fine grooves, more specifically the invention pertains to substrates having a large number of fine grooves, which are suitable for making an optical disk, a diffraction grating or the like, and a method of manufacturing such substrates.
2. Description of the Prior Art
Heretofore, it has been known that a substrate having a large number of fine grooves on its surface is used to make an optical disk, a diffraction grating or the like and variety of methods for manufacturing such substrate have already been proposed. For example, it is known that a substrate for use in making an optical disk or the like can be produced by applying a photoresist film on a substrate such as a glass plate, selectively exposing the photoresist film to light such as laser beam utilizing a laser exposure machine followed by developing the film and then processing the surface of the substrate according to an etching technique such as a dry etching method through the film serving as a mask.
On the other hand, fine patterns used for producing a diffraction grating can be produced according to a method such as that disclosed in Japanese Patent Unexamined Publication No. 60-21215 in which such pattern is manufactured by gelling a solution containing an organometal compound while bringing the solution into contact with a mold of a desired shape, releasing the gel from the mold and then drying it to cause shrinkage until a desired form is achieved.
The first photoresist technique makes it possible to precisely forming grooves, which are spaced several microns apart from one another, on a substrate such as a glass plate, while when selectively exposing a photoresist layer to laser beam, it takes a long period of time for exposure, for example, it takes several tens of minutes to expose only one disk-like substrate, which leads to the extreme reduction in productivity. Moreover, the exposure machine of high precision utilizing a laser is very expensive and requires tedious operations to assure the precision.
In addition, the method in which a solution of organometal compounds permits the formation of patterns more finer than those of the mold used due to the shrinkage of the gel. However, there exists different problems such that the resultant gel tends to cause cracks during shrinkage and in worst case the gel is broken and that the gel suffers deformation such as warpage because of nonuniform drying rate.
For the purpose of eliminating the problems accompanied by the foregoing methods, Japanese patent application Ser. No. 60-241193 discloses a method for producing substrates with fine grooves, which comprises the steps of forming a plastic film on a substrate by applying a solution containing an organometal compound, impressing the surface with a mold having ridges of a desired pattern to form grooves corresponding to the ridges of the mold onto the surface of the film and then calcining the coated film to solidify it.
According to this method for producing substrate with grooves employing mold impressing technique, substrates for use in making an optical disk, a diffraction grating or the like may easily be obtained by quite simple procedures Moreover, the method provides advantages that the resulting film has a shrinkage lower than that of the gel film and seldom causes cracks and/or warpage since the organometal compound if formed in a film on the substrate body.
However, the substrate body such as a glass plate in general has undulation ranging from about 10.sup.2 to 10.sup.3 nm which are spaced 100 to 1000 .mu.m apart from one another. Therefore, if it is intended to uniformly form a great deal of very fine patterns of 70 nm in depth and about 1 .mu.m in width on such a substrate, the mold should be pressed to the substrate under an extremely high pressure, otherwise the mold would not be brought into contact with the whole surface of the substrate intimately, in particular the coated film due to the presence of undulation or unevenness on the surface of the mold and substrate. In addition, there is a strong tendency to cause deformation in shape and/or formation of defectives because of troublesome procedures during impressing the coated film with the mold.
It is believed that the foregoing drawbacks associated with the conventional method can be eliminated by (i) previously treating the surface of the substrate and die for pressing so that the degree of evenness thereof falls within the range of not more than .+-.10 nm, according to, for instance, an abrasion technique, or (ii) impressing the substrate with the die for pressing under a pressure which may cause deformation of the substrate. However, the method (i) leads to an extreme increase in the processing cost, while the method (ii) also results in the substantial wear and deformation of the die used, which in turn leads to the increase in cost for die. Therefore, in this method, the surface of the substrate and die for pressing should likewise be made uniform.